1. Field of the Invention
This invention relates to optically coupled isolators, also known as optocouplers, photon couplers, and optoisolators. In particular, this invention relates to a structure for, and a method of manufacture of, an improved optically coupled isolator having relatively high electrical isolation.
2. Description of the Prior Art
Optically coupled isolators consist of two electronic circuits coupled together optically, rather than electrically. Electronic signals are transmitted across an isolation barrier between the two circuits by light, or photons, rather than by electrons. Typically, the isolator comprises a semiconductor emitter, such as a light-emitting diode, in the first circuit and arranged so that its light strikes a semiconductor photon detector, such as a phototransistor, in the second circuit. A transparent insulation fills the space between the emitter and detector, providing electrical isolation. Gallium-arsenide infrared emitters are often used because their 900-nanometer wave-length output falls near the maximum spectral response of the commonly used silicon phototransistor. As both the emitter and detector comprise semiconductors, the isolator is manufactured using standard semiconductor processing techniques, is relatively small in size, and is usually sealed in a small, standard size package.
Some applications for isolators include those where it is desirable to isolate electrically one circuit from another, such as in medical instrumentation. Other applications include those in which it is desirable to transmit an electronic signal between circuits while eliminating noise within the signal, such as in computers and other kinds of switching functions.
The level of applied voltage that can be handled by an isolator without electrical connection between circuits occurring is a function of the distance between the emitter and detector, and a function of the dielectric strength of the transparent insulator located in the space between the detector and emitter. With the need to manufacture isolators economically through the use of standard size packages, such as the small dual-inline package, one is limited in the length of the space available between the detector and emitter. Moreover, if the space becomes too long, the isolator would lose efficiency because of the loss of light energy between the emitter and detector, caused by diffraction, diffusion, reflection, and so forth. Typically, the detector surface facing the emitter is larger than the emitter surface facing the detector in order to ensure that more light will reach the detector. Consequently, for a given length of space between the emitter and detector, the dielectric strength of the insulator in the space determines the isolator's ability to withstand high levels of applied voltage and still maintain electrical isolation. Previously, various kinds of insulation material have been used in the space between the emitter and detector, for example, plastic film such as mylar, and plastic resins such as silicone and epoxy, all of which transmit up to about 95 percent or more of the applied light, and are suitable for semiconductor processing techniques. The typical dielectric strength of many of these materials is on the order of about 500 volts per mil, providing isolators capable of withstanding applied voltages of 2,500 to 3,500 volts. In order to increase the level of applied voltages that the isolator can withstand, it is desirable that the transparent insulation material in the space have a dielectric strength in the range of 1,000 volts per mil, or more, about twice that of the above-mentioned materials.
An insulation material that could be used in the space between the emitter and detector is glass, which transmits up to about 98 percent or more of the applied light and has high dielectric strength, such as on the order of 1,000 volts per mil. Unfortunately, glass is relatively rigid and difficult to process easily using standard semiconductor processing techniques for assembly of the isolators. Moreover, some type of special structure is necessary to support the glass firmly in place in the space between the emitter and detector, and to maintain the desired alignment during subsequent assembly and system use, particularly when sudden jolts or vigorous vibrations occur. Previously, one of several known metalization procedures has been used to provide areas on the glass that can be attached to some kind of a frame in the isolator. Metalization requires steps of deposition and chemical etching, often requiring the use of various chemicals, such as acids. Such chemical treatment can contaminate the glass, so that when the latter reaches a temperature of around 80.degree. F., residual metallic ions, such as sodium, are able to migrate from the glass surface into the detector which is in direct contact with the glass, rendering the detector incapable of functioning effectively in an isolator. Moreover, even if it were possible to thoroughly clean the glass surface of foreign ions by extensive rinsing in deionized water after the etching step, alkali ions present to some degree in any glass would be free to migrate into the detector structure under the influence of temperature and electric field, because of the direct contact between the glass and the detector.
Therefore, an improved structure, and method of making the structure, is needed wherein the transparent insulation material in the space between the emitter and the detector is of a relatively high dielectric strength, and is also compatible with standard semiconductor processing techniques so that the cost of manufacturing the isolator is not substantially increased.